Environmental Engineering Reference
In-Depth Information
than that of a pulverized coal-fired steam plant. At present, these costs are not offset by the higher
thermal efficiency.
In the United States, the Electric Power Research Institute together with a consortium of
private utilities built a demonstration IGCC power plant including an air separation unit in Barstow,
California, called the Coolwater Plant. It had a net electric power output of 105 MW, but because
not all components were optimized and integrated, its thermal efficiency was only 31%. The
air emissions met the stringent requirements of the California Air Resources Board. It operated
satisfactorily for 5 years (1984-1989), but was moth-balled thereafter because of the high operating
costs. However, the Coolwater Plant demonstrated that IGCC is feasible and can be compliant with
strict emission standards.
5.3.3
Cogeneration
Cogeneration is the term applied to systems that provide both electrical power and useful heat
from the burning of fuel. In industrial or commercial installations the heat may be used for space
heating or material processing. The incentive for cogeneration is primarily financial in that the cost
of supplying electricity and heat via a cogeneration scheme might be less than supplying them
separately, such as purchasing electric power from a supplier while generating heat from an in-
plant furnace or boiler. Whether or not a cogeneration system reduces the amount of fuel needed to
supply the electricity and heat depends upon the details of the cogeneration system. Also, emission
controls may be more efficient and cheaper on a large-scale electric power plant than on a smaller-
scale cogeneration plant—unless the latter is fueled by natural gas, which inherently produces less
pollutant emissions. In 2000, about 12% of U.S. electric generation capacity was in cogeneration
facilities.
When a heat engine drives an electric generator to produce electricity, it also provides a stream
of hot exhaust gas. Where the exhaust gas is warm enough to be used for process or space heat,
some of the exhaust gas enthalpy may be extracted to satisfy the heat requirement in a cogeneration
plant. If
Q
fuel
is the rate of fuel heat consumption needed to generate electric power
P
el
and process
heat
Q
pr oc
in a cogeneration plant, then
Q
fuel
=
P
el
+
Q
ex
(5.13)
P
el
=
η
th
Q
fuel
(5.14)
Q
proc
=
η
xch
Q
ex
=
η
xch
(
1
−
η
th
)
Q
fuel
(5.15)
where
Q
ex
is the enthalpy flux of the exhaust gas,
η
th
is the thermal efficiency of the electrical
η
xch
≤
generation process, and
1 is the fraction of the exhaust stream enthalpy that is delivered
as heat for processing. The split of fuel heat
Q
fuel
between electric power,
η
th
Q
fuel
, and process
heat,
η
xch
(
1
−
η
th
)
Q
fuel
, depends upon the thermal efficiency
η
th
of the heat engine and the heat
exchanger effectiveness
η
xch
. The latter depends principally on the temperature at which process
heat is delivered compared with the temperature of the exhaust gas—being greatest when the
difference is large, and least when it is small. When process heat is needed at high temperatures,
the usable process heat from a cogeneration system may be too small compared to the electric
power generated to justify this complex system, and it may be more economical and fuel efficient
